HELP! - CANT Send HTML E-mails

As an expert user of PC's I am stumped by this problem. A friend has a G4 MAC with OSX running. He has always had the same problem that he canít send HTML e-mails, only receive. We are not doing anything clever - he canít send clickable links in an e-mail. We have tried the inbuilt e-mail client, MS Entourage, Eudora and others. Obviously in each case we set the e-mail client to send HTML not plain text. Any ideas?